Fumiko Yonezawa

Glass Transition

Computer simulations of melting, crystallization, glass transition, and annealing for a model system of 864 Lennard-Jones (LJ) atoms under a periodic boundary condition are carried out using constant-pressure molec ular dynamics techniques with temperature control. When an fcc crystal of LJ atoms is heated, melting occurs; however, an LJ liquid, when quenched slowly, crystallizes into layers with stacking faults. Each layer forms a two-dimensional, close-packed structure with occasional point defects but without dislocations. When the quench rate is high enough, an LJ liquid transforms into a disordered structure without a discontinuous change in volume. The dependence of the glass transi tion on the quench rate is determined by examining macroscopically observable physical features. Several microscopic structure parameters are introduced to ana lyze, at the atomic level, the structures of glasses pro duced by different quench rates. When annealed, a glass made with a low enough quench rate is stable against crystallization. Identifying the number of atoms in the system having local icosahedral symmetry is a prom ising method of characterizing the glasss stability. The microscopic structural changes in the annealing pro cesses are presented in the companion video to this paper.

Isothermal–isobaric computer simulations of melting and crystallization of a Lennard‐Jones system

By means of constant‐temperature, constant‐pressure molecular dynamics techniques, we simulate the melting and crystallization processes of a model system composed of 864 Lennard‐Jones (LJ) particles under periodic boundary conditions. On heating an fcc crystal of LJ particles, it is ascertained that melting takes place. On the other hand, a LJ liquid, when quenched slowly, crystallizes into a stacking of layers with stacking faults where each layer forms a close‐packed structure with occasional point defects. The atomic configuration is not always nucleated into a completely ordered structure. A large hysteresis in the volume‐temperature curve is observed. The volume contraction at the transition is characterized by two different growth rates, relatively slow at the first stage and relatively fast at the final stage. The critical cooling rate which separates the crystal‐forming cooling rates and the glass‐forming cooling rates is between 4×1010 and 4×1011 K/s for argon. On taking advantage of computer si...

Liquid–vapor coexistence curves of several interatomic model potentials

We clarify the relation between an interatomic potentials and liquid–vapor critical points. For this purpose, we calculate the liquid–vapor coexistence curves of several interatomic model potentials such as the Lennard-Jones n−6 (n=7–32), the Morse, and the modified Stillinger–Weber potentials by the NpT plus test particle method. From these results, we find several universal properties irrespective of the potential type: (1) The law of rectilinear diameter is fulfilled in the density–temperature plane. (2) The coexistence curve scaled by the critical temperature and density almost coincides with one another. On the other hand, we also find some properties which are definitely potential dependent. In order to demonstrate this point, we introduce a new parameter a1=2π/3∫xmin∞x du(x)/dx x2 dx [x: reduced distance, xmin: the minimum position of a potential, u(x): reduced potential] which expresses the effect of the attractive force. By making use of this parameter, we find that the critical temperature Tc an...

Glass Transition and Relaxation of Disordered Structures

Publisher Summary This chapter discusses some outcomes from molecular dynamics simulations of the glass transition and of the annealing processes with a view to clarifying the nature of amorphous structures. Some of the properties have been found from the macroscopic and microscopic analyses of the atomic configurations. Melting on heating, crystallization on slow cooling, and a transformation to a glass on rapid cooling are observed in a system composed of Lennard–Jones (LJ) atoms. Relaxation processes in a supercooled liquid have been compared with those in a glass by laying special emphasis on the relationship between fluctuations and relaxations in disordered systems. The mechanism of the glass transition is essentially of a kinetic nature, which is reflected in most of the physical properties. Atoms in an amorphous system are classified according to the local symmetry around each atom. For instance, atoms with icosahedral symmetry are typical of amorphous structures. The chapter discusses that the validity of computer simulations is reinforced by minimizing the effects because of finite sizes and to the periodic boundary condition. The scope of the molecular dynamics methods is widened by extending the classes of subjects to be studied.

Isobaric-isothermal molecular dynamics study on the glass transition of a Lennard-Jones system

Abstract By means of constant-pressure, constant-temperature molecular dynamics simulations, we study the glass transition of a system composed of 864 Lennard-Jones particles with periodic boundary conditions. We calculate the thermodynamic properties, the structure properties, the diffusion constant and the microscopic structural parameters of our system, all of which show, at the nearly same temperature, the behaviours characteristic of the glass transition. The effect of the quench rate is such that the lower the quench rate, the lower the glass transition temperature and the more stable the obtained glass. Our simulations indicate that, for argon, the critical quench rate which separates the glass-forming quench rates and the crystal-forming quench rates is in the range between 4×10 10 K/sec and 40×10 11 K/sec.

Reliable Determination of the Liquid-Vapor Critical Point by the NVT Plus Test Particle Method.

We find out that the N V T plus test particle method is effective to obtain the reliable data for liquid-vapor critical point from microscopic information of interatomic potentials. The basic idea of this method is to calculate chemical potentials by test-particle insertions during canonical simulations, from which the equilibrium vapor pressure is determined. Simulations are performed at a fixed volume so that the undesirable volume fluctuations do not occur between the liquid and vapor phase. As a consequence it becomes possible to make the simulations stable even near the critical point. This is an outstanding advantage of the N V T plus test particle method when compared to the N p T plus test particle method. We apply the N V T plus test particle method to the Lennard-Jones 12–6 potential. We determine the coexistence curve far closer to the critical point than the result of the N p T plus test particle method. By applying the mixed-field finite-size scaling idea to our data, we obtain the critical t...

Confinement-Induced Stable Amorphous Solid of Lennard–Jones Argon

When a material is confined in a nanometer-scale pore, novel properties not seen in a macroscopic-scale material are expected. With this situation in mind, we carry out molecular dynamics (MD) simulation for the purpose of investigating the atomic configurations of Lennard–Jones (LJ) Ar solid in a pore of 3.4 nm in diameter. These results concerning the radial distribution function, Voronoi face parameters and bond-orientational order parameter show that the atomic structure of the confined Ar solid is non-crystalline (or in other words, amorphous) with a large number of Ar atoms having the local-icosahedral symmetry. The same kinds of the atomic configurations appears in bulk LJ glasses constructed via glass transition. However, there DOES exist a remarkable difference between these two cases. The amorphous structure in a bulk Ar glass easily crystallizes, after the annealing of 0.045–0.3 ns, into a closest-packed crystal (hcp, fcc or even a random stacking). On the other hand, the amorphous structure in...

Structural and Electronic Properties of Expanded Selenium in the Liquid-Vapor Supercritical Region

The purpose of this work is to give a theoretical explanation of the nonmetal-to-metal (NM-to-M) transition observed in liquid Se when the system is expanded by increasing temperature in the liquid-vapor supercritical region. The NM-to-M transition accompanying the expansion of the system is unexpected since it is in contradiction to the established understanding of NM-to-M transitions. We introduce structural models for expanded Se and calculate the electronic states and the wave functions of these models by the simulated annealing method. From the results of our calculations, we show that, when some of Se–Se bonds are weakened in the process of expanding the system or equivalently in the process of decreasing the density, the splitting between the bonding and anti-bonding level is reduced and consequently the anti-bonding band is relatively lowered, which leads to the disappearance of the band gap between the anti-bonding and lone-pair band, or in other words to the occurrence of the NM-to-M transition.

Nature of amorphous and liquid structures — computer simulations and statistical geometry

Abstract In order to characterize topologically-disordered systems at the atomic level, we synthesize several liquid and amorphous structures by means of molecular dynamics technique. We study the typical features of these structures by using the methods of statistical geometry such as the Voronoi-Delaunay analyses. We introduce several new ways of structure characterization.

Ab-initio molecular dynamics simulations of amorphous silicon

With a view to elucidating the microscopic mechanisms of the Staebler-Wronski effect in a-Si:H, we study the fundamental electronic and atomic processes by means of ab-initio molecular dynamics simulations originally proposed by Car and Parrinello. Our conclusions include: (1) Weak Si-Si bonds tend to be weakened further by illumination of light or by hole injection. (2) When a Si-Si bond is weak enough, there exists a metastable site for a hydrogen atom between these two Si atoms, at which the total energy becomes local minimum. The metastable site is either “ on ” or “ off ” Si-Si bond depending on the length of the Si-Si bond. (3) It requires much less energy to move H from a typical Si-H bond into an “ off-bond ” metastable site in the vicinity than into some interstitial site.